2 Answers
2

To help better understand the relationship of DPI to pixel dimensions, take an 800x600 pixel image for example:

Using 300dpi, an 800x600 image will print 2.6x2 inches.

Using 200dpi, an 800x600 image will print 4x3 inches.

Using 100dpi, an 800x600 image will print at 8x6 inches.

Notes:

As the DPI gets less dense, or lower, the print quality degrades.

The pixel dimensions don't change in the example, only the printed pixel density

Extremely large prints may be printed at lower DPI because they are viewed at a distance

When changing DPI in a photo editing program, you can choose to resample which will change pixel dimension, or, if you choose not to resample, the pixels remain unchanged, only the print dimensions change.

Thanks for a clear answer. Showing exactly how the DPI affects the pixel dimensions when printing helped! Is there a standard DPI I should use?
–
Canon GangstaAug 5 '10 at 15:14

@Canon Gangsta: The DPI setting you use affects the print quality, what you use depend on the printer. Note the difference between DPI (resolution of the printer output) and PPI (resolution of the image). A reasonable resolution for an image to be printed is 200 - 500 PPI.
–
GuffaAug 5 '10 at 15:28

It should be noted that the numbers quoted above (i.e. 300dpi) are actually the resolution, not the DPI. DPI and PPI are distinct factors, and depending on the type of printer used, may be identical or may be very different. DPI is the resolution of actual dots printed by the printer, and there may be many dots per pixel. Particularly with ink jet printers, DPI is usually much higher than PPI (i.e. Canon 9500 is 4800x1200 dpi, but native resolution of 600ppi). I would change DPI to PPI in all examples above.
–
jrista♦Sep 7 '10 at 4:44

It's used to descibe the output resolution of printers, and it's also often used instead of the PPI (Pixels Per Inch) unit, which is more appropriate for describing the resolution of computer screens, scanners and image files.

So, a printer might have a resolution of 2400 DPI, but that doesn't mean that you can print an image at 2400 PPI. The printer uses dots of only a few colors (usually cyan, magenta, yellow and black) which are combined in patterns to produce the entire spectrum in different intensities. A reasonable resolution for an image to be printed is 200 - 500 PPI.

If you for example have an image with the specified resolution 300 PPI, it will need 300 pixels for each inch when printed. To print as 4" x 6" the image needs to have the dimensions 1200 x 1800 pixels.

If you print this image using a printer setting of 1200 DPI or 2400 DPI, it will still be the same size, but the latter will have better quality because the printer fits more ink dots in the same area.

If you want to publish an image on the web, the PPI setting in the file has no relevance. The image is displayed with one image pixel per screen pixel. The resolution of a screen is usually around 100 PPI, so that's what people tend to specify when creating images for the web, eventhough the value is ignored by the browsers.